Real-world effectiveness of the influenza vaccine among school-aged children in Shenzhen, China, 2024-2025 season: A test-negative design study.

OBJECTIVE: To comprehensively evaluate the real-world effectiveness of the influenza vaccine among school-aged children across a dual-peak season, specifically examining subtype-specific protection, age variations, durability of protection over time, and the impact of repeated prior-season vaccination. METHODS: A test-negative design was used to assess vaccine effectiveness (VE) among school-aged children in Shenzhen. Based on questionnaire data and laboratory testing, patients with laboratory-confirmed influenza were classified as cases, while test-negative patients served as controls. Multivariable logistic regression was applied to compare the odds of vaccination between cases and controls. RESULTS: Among 1669 eligible participants (463 cases and 1206 test-negative controls), the overall adjusted VE against medically attended influenza was 46.36% (95% CI: 29.23-59.34%). VE differed across age groups and was mainly observed in younger children. Significant protection was observed within 14-119 days post-vaccination, became non-significant during 120-239 days, and re-emerged during the summer peak at 240-299 days. Prior-season vaccination alone provided no significant residual protection, while repeated vaccination across two consecutive seasons was associated with reduced effectiveness in the current season. Subtype-specific VE was 43.48% (95% CI: 18.00-61.04%) against influenza A(H1N1), 34.42% (95% CI: 0.55-56.76%) against influenza A(H3N2), and 56.95% (95% CI: 7.68-79.93%) against the B/Victoria lineage. During the winter-spring peak, VE was 49.84% (95% CI: 26.90-65.59%). CONCLUSIONS: Influenza vaccine provided moderate effectiveness against influenza-associated outpatient visits among school-aged children in Shenzhen during the 2024-2025 season.